It is known to employ a thermostatic capsule to move a shuttle to open or close small gaps on either side of the shuttle. It is convenient for this shuttle to be circular with the gaps being annular, but this gives rise to other problems.
The inlet to the annular gaps is conveniently radially inwards, with mixed flow then passing axially to the thermostatic capsule. This arrangement calls for flow regulation downstream of the capsule and on the side of the capsule opposite from flow and temperature controls. Conventionally, the flow control has been carried out by means of a member spaced radially outwards of the shuttle. This mitigates against a compact structure.